The relationship between potassic and calc-alkaline post-orogenic magmatism at Vico volcano, central Italy

Abstract

Post-orogenic magmatism in Italy took place during the Plio-Pleistocene. Alkaline ultrapotassic magmas and sub-alkaline magmas were erupted from closely-spaced volcanoes throughout the central part of Italy. Therefore, central Italy is one of the best site to study the relationship between calc-alkaline and alkaline magmatism in complex tectonic regimes. Activity at Vico volcano developed in three periods between 0.419 Ma and 0.095 Ma. Vico products overlie leucite-free volcanic rocks of Monte Cimino volcano that have a variable affinity from high-potassium calc-alkaline to ultrapotassic. Most of the Vico rocks are leucite-bearing Roman-type (ultrapotassic). However, during the first period, leucite-free silicic rocks are interbedded with leucite-bearing products. The early-erupted Vico magmas are isotopically distinct from the youngest magmas. Age-corrected Sr (Sr i =0.71110–0.71149) and Nd (Nd i =0.51210–0.51211) isotope ratios fall within the gap between Roman-type (including younger Vico) and Monte Cimino rocks. Some leucite-free Vico rocks contain orthopyroxene crystals and Al contents in clinopyroxenes are lower than those in clinopyroxenes from Roman-type rocks. These characteristics are, however, also found in Monte Cimino rocks. At the beginning of the first period, leucite-free latites to trachytes and rhyolites with high Sr i (0.71127–0.71149) were erupted. Trachytes and leucite-bearing latites with lower Sr i (0.71110–0.71138) were erupted later. The younger trachytes have higher Rb, and LRE element contents than older leucite-free rocks with similar SiO 2 contents. Furthermore, the onset of leucite crystallization is marked by a change in 87Sr/ 86Sr and SiO 2. Sr–isotope ratios and their time-related variations, as well as the presence of two different trends in some trace element variation diagrams, define two distinct modes of magma differentiation. Magmas evolved along a stratigraphically “lower trend” at the beginning of the activity whereas they evolved differently along a stratigraphically “upper trend” later in the volcanic history. Modelling of geochemical and Sr–isotope variations indicates that, along the lower trend, fractional crystallisation was the main evolutionary process, whereas along the upper trend mixing between a leucite-free magma and a leucite-bearing magma occurred during crystallisation. The Sr–isotope compositions of rocks from the early stage of activity at Vico suggest mixing between magmas having different petrological affinities: a leucite-bearing Roman-type magma and a high-potassium calc-alkaline magma.